Thallium Arsenates

Thallous Orthoarsenate, Tl₃AsO₄, has been obtained by the action of ammonia on a solution of the dihydrogen salt. It separates as white silky needles.

Thallous Monohydrogen Orthoarsenate, Tl₂HAsO₄, has been prepared by dissolving thallium in aqueous arsenic acid, or by saturating a boiling solution of the latter with thallous carbonate. It separates on cooling as long transparent needle-shaped crystals, soluble in water. When heated, the salt melts at 120° C. and at a higher temperature decomposes to form thallous oxide and arsenious oxide. Thallous dihydrogen orthoarsenate, TlH₂AsO₄, is formed by boiling thallic oxide in aqueous arsenious acid; on cooling the solution colourless glittering needle-shaped crystals separate. The salt is readily soluble in water and may be heated to 150° C. without decomposition.

Thallic Orthoarsenate, TlAsO₄.2H₂O, separates as a lemon-yellow gelatinous mass when aqueous arsenic acid is added to a solution of thallic nitrate. It is insoluble in boiling water, readily soluble in hydrochloric acid and decomposed by aqueous alkali or ammonia with formation of thallic hydroxide or a basic salt.